The invention relates to a data input device produced by means of a switch operated by a stem.
The invention is particularly applicable for a stem that can be pulled or pushed about a stable position. The two displacements of the stem, pull and push, can each be used to enter a data item. For each direction of displacement of the stem, the switch is generally placed at one end of the stem. An operator pulls or pushes the stem, causing the corresponding switch to be operated. The stem can also be used to operate a rotary coder wheel about the translation axis of the stem. The data obtained from the rotary coder wheel can be coded optically in an electronic part of the device.
In such a device, it is desirable to generate a tactile sensation, the main function of which is to ensure that the electrical triggering of the switch is ensured after passing a peak of force. For a good tactile sensation, it is necessary for the clearance to be significant, for example greater than a millimeter. Such devices are found in the aeronautical industry and more particularly in aircraft instrument panels. The flight safety of the aircraft may depend on the data input carried out by means of the device. This is why many manufacturers impose severe constraints in the tactile feedback that a pilot must feel when operating the device.
For this, the applicant has attempted to produce this function by means of a ball cooperating with the stem. More specifically, on the stem there is formed a cam comprising an inclined face followed by a plane parallel to the displacement axis of the stem. The ball can be displaced perpendicularly to the displacement axis of the stem by bearing on the cam by means of a spring. At rest, the stem is in a stable position. This position is held by a return spring that can be compressed along the displacement axis of the stem. In this stable position, the ball is in contact with the cam at the bottom of the inclined face. When the stem is actuated by an operator, the ball rises on the inclined face while compressing its spring until the parallel plane is reached. The force exerted by the ball on the cam is added to that exerted by the return spring of the stem. When the ball is in contact with the parallel plane, the force added by the ball, returned on the displacement axis of the stem, is almost zero, friction apart, and the only force that the operator has to overcome is that generated by the return spring of the stem. However, when the ball is in contact with the inclined face, it exerts an axial force component on the stem. This axial component, added to the force generated by the return spring, forms a peak of force that the operator must overcome by actuating the stem.
The accuracy of these systems depends notably on the diameter of the ball and its position on the inclined face in the stable position, which imposes tight production tolerances. The peak of force depends on the diameter of the ball, on the force exerted by the spring of the ball and on the slope of the inclined face. In a curve giving the force exerted on the stem as a function of the displacement of the latter, the slope of the return curve of the stem depends on the stiffness of the return spring and on the friction force of the ball on the cam.
These systems require ball diameters, a compression force of the balls and a length and height of the slope of the click that are significant to have a mechanical travel and a dip in force that are significant for large clearances with an identifiable tactile sensation. The significant compression forces of the balls demand superficially hard materials to sustain the wear of the repeated operations.
Furthermore, after numerous operations, the wear of the cam and of the ball affects the tactile sensation by increasing the depth of displacement between the maximum force at the peak and the minimum force that follows.
The invention aims to overcome all or some of the problems cited above.